1. Field of the Invention
The present invention relates to a light emitting tube and a light source apparatus using the same, which are arranged to efficiently use a light flux emitted from a light emitting part and to be suited to illumination apparatuses or projection apparatuses such as a liquid crystal projector, an overhead projector and the like.
2. Description of Related Art
A light emitting tube, such as a metal-halide lamp or a high-pressure mercury lamp, has heretofore been used for the light source apparatuses such as a liquid crystal projector.
FIG. 16 schematically shows the essential parts of a light emitting tube used for a conventional liquid crystal projector. In FIG. 16, reference numeral 210 denotes the light emitting tube. Two arc bars 202 are disposed in the light emitting tube 210. A middle point of a space between the two arc bars 202 is a light emission center 202a. The light emitting tube 210 is provided with a spherical body part 210a. The spherical body part 210a has an outer wall 210d and an inner wall 210b. The light emission center 202a is made to be located at a sphere center 210c of the outer wall 210d of the spherical body part 210a.
Referring to FIG. 16, a light flux radiating from the light emission center 202a exits through the inner wall 210b from the spherical body part 210a. In this instance, the distribution angle of luminous intensity of the light flux exiting from the light emitting tube 210 is wide and has a luminous intensity distribution which is symmetrical at about 50 degrees with respect to a line perpendicular to the longitudinal direction of the light emitting tube 210 indicated by an double-headed arrow 203, as shown in FIG. 16.
In a case where the light emitting tube 210 having a symmetrical light intensity distribution characteristic as shown in FIG. 16 is used in combination, for example, with an ellipsoidal mirror 211 which is employed as a light-condensing reflecting mirror, the ellipsoidal mirror 211 must be in such a shape that it has a small first focal point F1, as shown in FIG. 17, in order to use as much as possible the light emitted from the light emitting tube 210. As a result, the ellipsoidal mirror 211 comes to have a large image-forming magnifying rate. However, in a case where an aperture provided for incidence of an illumination light is small and thus necessitates a small light source image, the large image-magnifying rate of the ellipsoidal mirror 211 makes it difficult to enhance the efficiency of illumination.
In particular, rays of light radiating at angles more than 50 degrees with respect to a line perpendicular to the longitudinal direction of the light emitting tube 210 are repeatedly total-reflected by the inner wall surface of the light emitting tube 210 to be caused to exit from the end face of the light emitting tube 210. In other words, rays of light radiating at angles more than 50 degrees with respect to a line perpendicular to the longitudinal direction of the light emitting tube 210 fail to be projected on the reflection surface of the ellipsoidal mirror 211 and thus become wasteful light.
Further, in a case where the light emitting tube 210 having a symmetrical light intensity distribution characteristic as shown in FIG. 16 is used in combination, for example, with a paraboloidal mirror 212 which is employed as a light-condensing reflecting mirror, the paraboloidal mirror 212 must be in such a shape that has a short focal point F, as shown in FIG. 18, in order to use as much as possible the light emitted from the light emitting tube 210. Such an arrangement, however, causes rays of light reflected by the paraboloidal mirror 212 to have an inadequate degree of parallelism.
The degree of parallelism of the exiting light flux greatly degrades particularly at a part of the reflection surface of the paraboloidal mirror 212 in the neighborhood of the optical axis A, because the angle subtended by the light emission center 202a is large. Besides, like in the case of the ellipsoidal mirror shown in FIG. 17, rays of light radiating at angles more than 50 degrees with respect to a line perpendicular to the longitudinal direction of the light emitting tube 210 are repeatedly total-reflected by the inner wall surface of the light emitting tube 210 and eventually exit from the end face of the light emitting tube 210. In other words, rays of light radiating at angles more than 50 degrees with respect to a line perpendicular to the longitudinal direction of the light emitting tube 210 fail to be projected on the reflection surface of the paraboloidal mirror 212 and thus become wasteful light.